This invention relates to a modulation system for use in carrying out linear modulation in a radio communication system for digital signal transmission or analog signal transmission. It is to be noted throughout the instant specification that quadrature amplitude-phase modulation is exemplified as the linear modulation although this invention is not restricted to the quadrature amplitude-phase modulation.
Recently, deficiency or lack of a radio wave resource, namely, a radio frequency band has been pointed out and awakens a new interest in narrowing a radio frequency bandwidth for each radio channel so as to achieve effective use of the radio frequency band. In the light of narrowing the radio frequency bandwidth for each radio channel, linear modulation is superior to nonlinear modulation, such as frequency modulation, because the latter brings about inevitable expansion of the radio frequency bandwidth for each radio channel.
In general, a modulation system for such linear modulation is for use in combination with an amplifier, such as a solid state power amplifier or a traveling wave tube (TWT) amplifier, which inevitably has nonlinearities in phase and amplitude. The nonlinearities of the amplifier come from nonlinear AM/AM and AM/PM conversion characteristics and give rise to degradation of a transmission spectrum of a transmission signal produced by the amplifier and to deterioration of a reception characteristic of a receiver for receiving the transmission signal. In addition, the nonlinearities cause an intermodulation component to occur and bring about interference between adjacent ones of the radio channels.
As will later be described with reference to one of nine drawing figures of the accompanying drawing, a modulation system is disclosed in U.S. Pat. No. 4,700,151 issued to Yoshinori Nagata, the instant applicant and assigned to the present assignee. According to the Nagata patent, the modulation system is for supplying, in response to a sampled signal of a sampling rate, a modulated analog signal to an amplifier having nonlinearities to make the amplifier produce an amplified output signal. The modulation system includes a subtracter for producing a difference signal representative of a difference between the sampled signal and an additional digital signal. Connected to the subtractor, a processing arrangement processes the sampled signal with reference to the difference signal to compensate for the nonlinearities to produce a processed signal. Connected to the processing arrangement, a modulating arrangement modulates the processed signal into the modulated analog signal. A demodulator is connected to the amplifier to demodulate the amplified output signal into a demodulated signal. Connected to the demodulator, an analog-to-digital converter converts the demodulated signal into the additional digital signal at a specific rate which is equal to the sampling rate.
With this structure, a delay characteristic is inevitably varied in actual usage of the modulation system in a feedback portion which includes the modulating arrangement, the amplifier, the demodulator, and the analog-to-digital converter. For example, whenever an actual selected channel is changed from one of communication channels to another of the communication channels in a mobile telephone system, a delay time is varied.
When the delay characteristic is varied in actual usage, it is impossible to accurately compensate for the nonlinearities of the amplifier. This is because the subtractor can not be supplied with a correct signal as the additional digital signal from the analog-to-digital converter.
In order to compensate for such variation of the delay characteristic, another modulation system is disclosed by Y. Furuya in Japanese Unexamined Patent Prepublication (Kokai) No. Syo 63-208,330, namely, No. 208,330/1988. The modulation system comprises a variable delaying circuit for delaying the sampled signal by a variable delay which is varied in accordance with a delay control signal. The variable delaying circuit thereby produces a delayed signal. A differentiating circuit produces a differential signal representative of a differential of the delayed signal. A divider produces a quotient signal representative of a quotient of the difference signal by the differential signal. A low-pass filter filters the quotient signal into a filtered signal. Responsive to the filtered signal, a control signal producing circuit produces the delay control signal by which the variable delay is determined.
The modulation system can compensate for the variation of the delay characteristic. However, it is impossible to rapidly deal with the variation of the delay characteristic. This is because the differentiating circuit does not differentiate the sampled signal but differentiates the delayed signal which is delayed by the delaying circuit. That is, no consideration is paid to rapidly compensate for the nonlinearities in the modulation device.